Clock control method and circuit

ABSTRACT

A clock control circuit for reducing jitter has at least one averaging circuit for generating, and outputting from an output terminal, a signal having a time difference obtained by internally dividing a time difference between first and second signals input respectively from first and second input terminals. First and second clock signals are supplied respectively to the first and second input terminals of the timing averaging circuit, and a clock in which a time difference between pulses of the first and second clock signals is averaged is generated.

STATUS OF RELATED APPLICATIONS

This application is a DIV of Ser. No. 10/627,632, filed Jul. 28, 2003,now U.S. Pat. No. 7,170,333, which is a DIV of Ser. No. 09/755,152,filed Jan. 8, 2001, now U.S. Pat. No. 6,600,354 and which is a DIV ofSer. No. 09/330,162, filed Jun. 11, 1999, now U.S. Pat. No. 6,333,659,the entire disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a method of controlling clock signals and to aclock control circuit. More particularly, the invention relates to aclock signal control method and circuit ideal for application to asemiconductor integrated circuit device synchronized to a system clockto control internal circuitry. More specifically, the invention relatesto a circuit that uses a timing averaging circuit for clock control, aswell as to a clock control circuit that uses a timing averaging circuitin a synchronizing circuit such as a delay-locked loop, phasesynchronizing loop or synchronous delay circuit.

DESCRIPTION OF THE RELATED ART

In a semiconductor integrated circuit synchronized to a system clock tocontrol internal circuitry, the entirety of the internal circuitry iscontrolled by executing a given circuit operation every cycle of theclock.

In order to assure operation that takes variance due to system clockjitter into account in a semiconductor integrated circuit of this kind,the length of time in clock cycle that can actually be used for circuitoperation is obtained by subtracting jitter time from the period of theclock. Accordingly, if we let Tmin represent the minimum time necessaryfor circuit operation executed in one clock cycle, then it will benecessary for the minimum period tCKmin of the clock to be set to a timeTmin+Tjitter, namely a length of time obtained by adding jitter timeTjitter to Tmin, as shown in FIG. 16.

Further, in order to reduce delay time between the system clock and aninternal clock or to multiply the frequency of the clock in asemiconductor integrated circuit synchronized to a system clock tocontrol the internal circuitry, the conventional practice is to use aphase-locked loop (PLL), a delay-locked loop (DLL) or a synchronousdelay circuit. However, these clock control circuits can be a source ofclock jitter and are susceptible to jitter of the system clock, in whichcase locking time is prolonged. Thus, these circuits tend to degrade thesynchronization characteristic.

A PLL is effective in reducing jitter depending upon how the PLL is setup. In a PLL, a clock having a frequency and phase the same as those ofan external clock is generated by a voltage-controlled oscillator (VCO)105 configured for feedback, as shown in FIG. 21. In this arrangement,the jitter component of the system clock is suppressed by a phasecomparator 102, a charge pump 103 connected to the output of the phasecomparator 102 and a loop filter 104 connected to the output of thecharge pump 103, thus making it possible to reduce jitter of the clockgenerated by the VCO 105. The charge pump 103 receives the output (UPand DOWN signals, etc.) from the phase comparator 102 and the outputnode thereof is charged or discharged, whereby a voltage correspondingto the phase difference between the clock and the output of the VCO 105is applied as the input voltage of the loop filter 104.

Because a PLL is a feedback circuit, however, a long period of time onthe order of several hundred to several thousand cycles is requireduntil the clock stabilizes. In addition, if jitter is too large, thereis the possibility that the PLL will not remain locked.

In a DLL, on the other hand, a clock having a phase the same as those ofan external clock is generated by a voltage-controlled delay circuit 115configured for feedback, as shown in FIG. 22. A problem that arises,therefore, is that external clock jitter passes through the delaycircuit as is and is transmitted to the internal circuitry from a clockdriver 106.

In a synchronous delay circuit, as shown in FIG. 23, a pair of delaycircuit chains 901, 902 and a dummy delay circuit 905, which comprisesan input buffer dummy 905A and a clock driver dummy 905B, are used tosubtract the delay time (td1+td2) of the dummy delay circuit 905 fromthe clock period tCK of a clock whose phase is the same as that of anexternal clock, whereby a delay quantity tV is obtained. The delayquantity tV is measured as the length of time of travel through thedelay circuit chain 901, and the delay is reproduced by the other delaycircuit chain 902, thereby synchronizing the internal clock to theexternal clock.

The synchronous delay circuit, which eliminates clock skew in a shortsynchronization time, finds use in high-speed clock synchronizingcircuits thanks to the simplicity of the circuitry and the low powerconsumption. Reference is had to the following literature which citesexamples of synchronous delay circuits of this kind:

[1] The specification of Japanese Patent Application Laid-Open (KOKAI)No. 8-237091;

[2] Jin-Man Han et al., “Skew Minimization Technique for 256M-bitSynchronous DRAM and beyond.” 1996 Symp. on VLSI Circ. pp. 192–193;

[3] Richard B. Watson et al., Clock Buffer Chip with Absolute DelayRegulation Over Process and Environment Variations.” Proc. of IEEE 1992CICC (Custom Integrated Circuits Conference), 25.2; and

[4] Yoshihiro OKAJIMA et al, “Digital Delay Locked Loop and DesignTechnique for High-Speed Synchronous Interface.” IEICE TRANS. ELECTRON.,VOL. E79-C, NO. 6 JUN. 1996 pp. 798–807.

As shown in FIG. 23, the synchronous delay circuit has a basic structurewhich includes a set of delay circuits, namely the delay circuit 901,which is used to measure a fixed time difference, and the dummy delaycircuit 905 providing a delay time corresponding to the delay timetd1+td2, which is obtained by adding the delay times td1 and td2 of aninput buffer 903 and clock driver 904, respectively.

In order for the dummy delay circuit 905 to make the delay time equal tothe sum of the delay times td1 and td2 of the input buffer 903 and clockdriver 904, respectively, there are many cases where the dummy delaycircuit 905 is constructed using the input buffer dummy 905A, whichcomprises circuitry exactly identical with that of the input buffer, andthe clock driver dummy 905B.

The delay circuit 901 and delay circuit 902 each consist of delaycircuits having equal delay times.

The purpose of the delay circuit 901 is to measure a fixed period oftime, and the purpose of the delay circuit 902 is to reproduce thisperiod of time. These objects can be achieved by passing the signalthrough the delay circuit 901 for the measured period of time andarranging it so that the signal can be passed through the delay circuit902 using a number of delay elements equivalent to the number of delayelements traversed by the signal in the delay circuit 901.

The following approach may be adopted to enable a signal to be passedthrough the delay circuit 902 using a number of delay elementsequivalent to the number of delay elements traversed by the signal inthe delay circuit 901. The delay circuit 901 and delay circuit 902 aredivided into two types depending upon their signal propagationdirections and, in order to decide the length of the delay circuit 902,the delay circuits are divided into two types depending upon whether theterminus is selected or the entire path of the circuit is selected,giving four types of delay circuits.

Specifically, if the delay circuit 901 and delay circuit 902 are dividedinto two types depending upon their signal propagation directions, thedelay circuits 901, 902 can have the same direction, with the length ofthe delay circuit 902 being decided on the side of its output terminalsin order to decide the number of elements in the delay circuit 902, asillustrated in FIGS. 26 and 27, or the delay circuits 901, 902 can haveopposite signal propagation directions, with the length of the delaycircuit 902 being decided on the side of its input terminals in order todecide the number of elements in the delay circuit 902, as depicted inFIGS. 24 and 25.

Further, in order to decide the length of the delay circuit 902, thereare two types of arrangements, namely one in which the terminus isselected and one in which the entire length of the delay circuit isselected, i.e., two approaches are available, namely one in which theterminus is selected, as shown in FIGS. 24 and 27, and one in which theentire path of the delay circuit is selected, as shown in FIGS. 25 and26.

It should be noted that FIG. 24 corresponds to the approach described inReference [1], namely the specification of Japanese Patent ApplicationLaid-Open (KOKAI) No. 8-237091, the application for which was filed bythe present inventor.

The arrangement illustrated in FIG. 25 corresponds to the approachdescribed in Reference [4] (IEICE TRANS. ELECTRON., VOL. E79-C, NO. 6JUN. 1996 pp. 798–807).

The arrangement illustrated in FIG. 26 corresponds to the approachdescribed in Reference [2] (1996 Symp. on VLSI Circ. pp. 192–193).

The arrangement illustrated in FIG. 27 corresponds to the approachdescribed in Reference [3] (Proc. of IEEE 1992 CICC 25.2) and Reference[4] (1996 Symp. on VLSI Circ. pp. 112–113).

SUMARRY OF THE DISCLOSURE

For the discussion, reference will be had to the schematic views andtiming charts shown in FIGS. 28 and 29 to describe an operation throughwhich clock skew is eliminated.

(1) Clock Delay When Synchronous Delay Circuit is Not Used

FIG. 28 illustrates an instance where a synchronous delay circuit is notused. As shown at (a) in FIG. 28, an external clock 906 is utilized asan internal clock 907 upon being directed through the input buffer 903and clock driver 904. The delay time difference between the external andinternal clocks is decided by the delay time td1 of the input buffer 903and the delay time td2 of the clock driver 904. The sum of these delaytimes, namely td1+td2, is the clock skew.

(2) Principle Underlying Elimination of Clock Delay When SynchronousDelay Circuit is Used

In order to eliminate clock skew effectively, a synchronous delaycircuit utilizes the fact that a clock pulse enters at the clock periodtCK. More specifically, a delay circuit providing a delay time oftCK−(td1+td2) is disposed between the input buffer (delay time td1) andthe clock driver (delay time td2) and it is so arranged that the sum ofthe delay times will be equal to the clock periodtCK[=td1+tCK−(td1+td2)+td2]. As a result, the timing of the internalclock output from the clock driver becomes equal to the timing of theexternal clock.

(3) Method of Eliminating Clock Delay When Synchronous Delay Circuit isUsed

A timing chart illustrating signal timing when a illustrating signaltiming when a synchronous delay circuit is acutally used is illustratedin FIG. 29.

The operation of the synchronous delay circuit requires two cycles.

The first cycle is used to measure the delay time tCK−(td1+td2), whichis dependent upon the period of the clock, and to decide the length ofthe delay of the delay circuit that reproduces the delay quantitytCK−(td1+td2).

The next cycle is used to apply the delay quantity tCK−(td1+td2).

In the first cycle, the dummy delay circuit 905 of the clock driver 904and the delay circuit 901 are used to measure the delay timetCK−(td1+td2) dependent upon the clock cycle.

The output of the input buffer 903, namely the first pulse of twosuccessive pulses in the external clock 906, is allowed to traverse thedummy delay circuit 905 and delay circuit 901 during one clock periodtCK, which lasts until the second pulse is output from the input buffer903. Since the delay time of the dummy delay circuit 905 is td1+td2, thetime required for a pulse to propagate through the delay circuit 901 istCK−(td1+td2).

The delay time of the delay circuit 902 is set so as to be equal to thetime tCK−(td1+td2) required for a pulse to travel through the delaycircuit 901.

Methods of setting the delay time of the delay circuit 902 can beclassified into the four types mentioned above, and each makes itpossible to attain the desired objective.

In the next cycle, the clock that emerges from the input buffer 903passes through the delay circuit 902 of delay tCK−(td1+td2) and exitsfrom the clock driver 904 to generate the internal clock 907 whose delayquantity is exactly the clock cycle tCK.

The process described above provide the internal clock 907, which isfree of clock skew, in two clock cycles.

Following problems have been encountered in the course of theinvestigations toward the present invention.

Thus, in order to reduce external clock jitter before the external clockis used as the internal clock in the conventional clock control circuit,a feedback circuit such as a PLL is required. The feedback circuit needsa long clock period in order for the clock to stabilize and it isdifficult, therefore, to attain a high-speed response. Another problemis that the synchronization characteristic is degraded by jitter.

A problem encountered with a DLL is that external clock jitterpropagates intact into the internal circuitry through the delay circuit.

A synchronous delay circuit is disadvantageous in that this circuit alsocauses an increase in external clock jitter.

Accordingly, an object of the present invention is to provide a clockcontrol circuit and control method for reducing jitter.

Another object of the present invention is to provide a delay-lockedloop circuit, a phase synchronizing loop and a synchronous delaycircuit.

According to the present invention, the foregoing objects are attainedby providing a clock control circuit having a timing averaging circuitgenerating a signal having a time component obtained by averaging aninput time difference between two signals input with a fixed timedifference between them, and means (or stage) which supplies the timingaveraging circuit with different pulses of a clock signal, wherein atime difference between the different pulses is subjected to internaldivision.

Further, the present invention provides a clock control circuit having atiming averaging circuit generating a signal having a time componentobtained by averaging an input time difference between two signals inputwith a fixed time difference between them, and a circuit supplying thetiming averaging circuit with different pulses of a clock signal,wherein a DLL circuit is equipped with a circuit internally dividing atime difference between these different pulses, the circuit having anexternal clock input and an internal clock input used in a phasecomparison, the output of the circuit being applied to avoltage-controlled delay circuit.

Further, the present invention provides a synchronous delay circuit forcontrolling a clock signal, comprising: a first delay circuit chainsthrough which a pulse or a pulse edge is caused to travel for a fixedperiod of time; a second delay circuit chains, to which a signal fromthe first delay circuit chains is input, that is capable of passing apulse or pulse edge along a length proportional to the length alongwhich the pulse or pulse edge traveled through the first delay circuitchains; a clock driver for outputting an internal clock from an outputof the second delay circuit chains; and a timing averaging circuit, towhich are input a clock signal from an input buffer and an output froman input buffer dummy having a delay time equivalent to that of theinput buffer on the internal clock signal output via the clock driver,for generating and outputting a signal having a time difference obtainedby internally dividing the time difference between these two signals,wherein an output of the timing averaging circuit chains is supplied tothe first delay circuit chains via the dummy delay circuit.

It should be noted that an arrangement may be adopted in which travel ofthe clock through the first delay line is halted for a period of timeequivalent to time required for travel through through input bufferdummy and clock driver.

Further aspects of the present invention are disclosed in the entireclaims which constitute part of the entire disclosure and may beunderstood in conjunction with the disclosure of the embodiments.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram useful in describing an embodiment of thepresent invention;

FIG. 2 is a timing chart useful in describing the principle of operationof an embodiment of the present invention;

FIG. 3 is a block diagram useful in describing the construction of anembodiment according to the present invention;

FIGS. 4( a) to 4(c) are diagrams useful in describing an embodiment ofthe present invention;

FIGS. 5( a) and 5(b) illustrate the construction of an embodiment of thepresent invention;

FIG. 6 illustrates the construction of an embodiment of the presentinvention;

FIG. 7 is a timing chart useful in describing the operation of anembodiment of the present invention;

FIG. 8 is a block diagram useful in describing the construction of anembodiment according to the present invention;

FIG. 9 is a diagram useful in describing the construction of anembodiment according to the present invention;

FIG. 10 is a signal waveform diagram showing the effects of a simulationaccording to an embodiment of the present invention;

FIG. 11 is a signal waveform diagram showing the effects of a simulationaccording to an embodiment of the present invention;

FIG. 12 is a block diagram showing the construction of an embodiment ofthe present invention;

FIG. 13 is a diagram showing the construction of a fixed phase delaycircuit according to an embodiment of the present invention;

FIG. 14 is a diagram showing the construction of an embodiment of thepresent invention;

FIG. 15 is a block diagram showing the construction of an embodiment ofthe present invention;

FIG. 16 is a timing chart for describing the prior art;

FIG. 17 is a block diagram showing the construction of a DLL accordingto an embodiment of the present invention;

FIG. 18 is a timing chart for describing the operation of an embodimentof the present invention;

FIG. 19 is a diagram showing the construction of an averaging circuitaccording to an embodiment of the present invention;

FIG. 20 is a block diagram showing the construction of a DLL accordingto an embodiment of the present invention;

FIG. 21 is a block diagram showing the construction of a PLL accordingto a conventional manner;

FIG. 22 is a block diagram showing the construction of a PLL accordingto a conventional manner;

FIGS. 23( a) and 23(b) are diagrams showing the construction of a clockcontrol circuit using a synchronous delay circuit according to a basicstructure.

FIG. 24 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to a basic structure;

FIG. 25 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to a basic structure;

FIG. 26 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to a basic structure;

FIG. 27 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to a basic structure;

FIGS. 28( a) and 28(b) are diagrams showing the construction of a clockcontrol circuit using a synchronous delay circuit according to a basicstructure;

FIGS. 29( a) and 29(b) are diagrams showing the construction of a clockcontrol circuit using a synchronous delay circuit according to a basicstructure;

FIG. 30 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 31 is a timing chart useful in describing the operation of a clockcontrol circuit using a synchronous delay circuit according to anembodiment of the present invention;

FIG. 32 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 33 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 34 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 35 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 36 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIGS. 37( a) and 37(b) are diagrams showing the construction of a clockcontrol circuit using a synchronous delay circuit according to anembodiment of the present invention;

FIG. 38 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 39 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 40 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 41 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 42 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 43 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 44 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 45 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 46 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 47 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 48 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 49 is a diagram showing the construction of a clock control circuitusing a synchronous delay circuit according to an embodiment of thepresent invention;

FIG. 50 is a diagram showing the construction of a synchronous delaycircuit having a timing averaging circuit according to an embodiment ofthe present invention;

FIG. 51 is a diagram showing the construction of a synchronous delaycircuit having a timing averaging circuit according to an embodiment ofthe present invention; and

FIG. 52 is a block diagram showing the construction of a PLL accordingto an embodiment of the present invention.

Embodiments of FIGS. 23 to 29 substantially correspond to FIGS. 9–15 ofcopending U.S. patent application Ser. No. 09/087,864 filed on Jun. 1,1998 by the same inventor (applicant) as the present invention, theentire disclosure whereof is incorporated herein by reference.

PREFERRED EMBODIMENTS OF THE INVENTION

Modes for practicing the present invention will now be described.

[First Mode of the Present Invention]

As shown in FIG. 1, a first preferred mode for practicing the presentinvention has a timing averaging circuit 1 for averaging the cyclesbetween pulses of a clock signal from an input buffer 3 (or clockdividing circuit), and a timing averaging circuit 2 connected, incascade form, to the output of the timing averaging circuit 1 asnecessary.

FIG. 2 shows the waveforms of basic operating signals useful indescribing the principle of operation of this mode of the presentinvention. If a clock 2-2 with a jitter component “a” with respect to anormal clock (2-1) enters, the jitter component has cycles of tCK+a andtCK−a with respect to the cycle tCK of the normal clock (2-1).

The jitter component of a primary jitter-compensated clock (2-3), whichis obtained by passing the clock (2-2) with jitter through the firsttiming averaging circuit 1 of FIG. 1, is averaged along the time axis,as a result of which the jitter component becomes 0.5a.

The jitter component of a secondary jitter-compensated clock (2-4),which is obtained by passing the primary jitter-compensated clock (2-3)through the second timing averaging circuit 2, becomes 0.25a.

Thus, jitter is compensated for by a timing averaging circuit foraveraging the cycles of the clock, and a plurality of such timingaveraging circuits are connected in series to double (or amplify) theeffectiveness of jitter suppression.

Various embodiments related to this mode for practicing the presentinvention will now be described.

Embodiment 1-1

FIG. 3 is a diagram useful in describing a first embodiment of thepresent invention. According to this embodiment, averaging of the timingof a clock signal is performed using a 2-phase clock. To accomplishthis, a frequency divider (a divide-by-two divider) 300 for generating a2-phase clock is disposed in forestage of timing averaging circuits 301,302. In order to average the timings of the edges of the 2-phase clock,the timing averaging circuit 301 is composed of timing dividing circuits304-1, 304-2 arranged in parallel in a number equal to the number ofclock phases. Similarly, the timing averaging circuit 302 is constitutedby timing dividing circuits 304-3, 304-4 arranged in parallel. Theoutputs of the timing dividing circuits 304-1, 304-2 are input to thetiming dividing circuits 304-3, 304-4, which proceed to output signalsrepresenting the averaged timings of these input signals. The outputs ofthe timing dividing circuits 304-3, 304-4 of timing averaging circuit302 output upon being multiplexed by a multiplexer circuit 305. Thus, aclock signal whose frequency is identical with that of the input clockis obtained as an output.

FIG. 4 is a diagram useful in describing the principle of operation of atiming dividing circuit (TMD) according to this embodiment of thepresent invention, and FIG. 5 is a diagram illustrating an example ofthe construction of the timing dividing circuit (TMD) according to thisembodiment of the present invention.

According to this embodiment of the present invention, as shown in FIG.5, first and second inputs IN1, IN2 are inverted by inverters INV1,INV2, and the outputs of the inverters INV1, INV2 are applied to thegates of PMOS transistors MP1, MP2, respectively, the drains of whichare tied together and the sources of which are connected to a powersupply VCC. The drains of the PMOS transistors MP1, MP2 are connected tothe drain of a grounded-source NMOS transistor MN1 and to a capacitor C(or to a load capacitance at the output). Connected to the gate of theNMOS transistor MN1 is the output of a NOR gate NOR, the inputs to whichare the first and second inputs IN1, IN2. The potential at the junction(output node) of the PMOS transistors MP1, MP2 and the NMOS transistorMN1 is output as a logic signal to an output terminal OUT through anoutput buffer BUF.

The PMOS transistor MP1 is turned on by the rising edge of the firstinput signal IN1, the timing of the positive-going transition of whichis earlier among those of the first and second input signal IN2, as aresult of which a current from the power supply flows through the PMOStransistor MP1 to charge the capacitor C (or the load capacitance).Next, the PMOS transistor MP2 is turned on by the transition (risingedge) of the second input signal IN2, as a result of which the capacitorC is charged through both of the conducting PMOS transistors MP1, MP2.The logical output OUT is produced at the threshold-value voltage of thebuffer BUF, whose input is the terminal voltage of the capacitor C (orthe load capacitance of the output node). With respect to the risingedge of the first input signal IN1, the signal OUT has a time differenceobtained by internally dividing the time difference between the firstand second input signals IN1, IN2.

The principle of timing division according to this embodiment of thepresent invention is as illustrated at (c) in FIG. 4. As shown at (a) inFIG. 4, three timing dividing circuits (TMD) having the constructionshown in FIG. 5 are provided with respect to the two clock inputs IN1,IN2 having a time difference between them. If the signal IN1 is appliedto both of the first and second input terminals of the first TMD, thesignals IN1 and IN2 are applied to respective ones of the first andsecond input terminals of the second TMD, and the signal IN2 is appliedto both of the first and second input terminals of the third TMD, thenthe signal waveforms of outputs OUT1 to OUT3 of the first to third TMDswill be as shown at (b) of FIG. 4.

Thus, it will be understood that a time difference T (equal to theperiod of the clock) exists between the rising edges of the signals IN1and IN2, and that the output OUT2 of the second TMD, whose inputs arethe signals IN1, IN2, appears at a timing approximately midway betweenthe output OUT1 of the first TMD and the output OUT3 of the third TMD.

The signal waveforms A1, A2 and A3 at FIG. 4( b) represent the signalwaveforms of the terminal potential of capacitor C produced by chargingof the capacitor C in FIG. 5. Outputs obtained by converting thesesignal waveforms A1, A2 and A3 to logic values at the buffer are thesignals OUT1, OUT2 and OUT3, respectively. The signal waveforms A1 andA3 indicate the terminal voltage of the capacitor C in a state in whichthe PMOS transistors MP1 and MP2 of FIG. 5 are conducting simultaneouslyto charge the capacitor C by a current equal to i1+i2. The signalwaveform A2 indicates the terminal voltage of the capacitor C in a casewhere the PMOS transistor MP1 of FIG. 5 is turned on to charge thecapacitor C by current i1 over a first period of time T (the gradient ofthe signal waveform A2 over this period of time is smaller than thegradients of the signal waveforms A1 and A3), after which the PMOStransistor MP2 is turned on to charge the capacitor C by the currenti1+i2 (the gradient of the signal waveform A2 over this second period oftime is the same as the gradients of the signal waveforms A1 and A3).

In other words, with reference to FIG. 5, first the capacitor C ischarged by the PMOS transistor MP1 for the first period of time equal tothe clock period T, then the capacitor C is charged by the PMOStransistors MP1 and MP2, thereby producing a time difference of T/2(t2=T/2+t1) in comparison with a case where the capacitor is charged byboth PMOS transistors MP1 and MP2 from the start. Furthermore, asindicated in FIG. 4( c), we have the following:t1=C*V/(i1+i2)t2=T+(C*V−i1*T)/(i1+i2)=T[i2/(i1+i2)]+t1t3=T+C*V/(i1+i2)When i1=i2 holds, we have t2=t1+T/2, where T represents the clock period(the time difference between the signals IN1 and IN2), C the capacitanceof the capacitor C, V the voltage across the capacitor C, and i1, i2 thecurrents that flow when the PMOS transistors MP1, MP2 turn on.

Embodiment 1-2

FIG. 6 is a diagram illustrating an example of the construction of thetiming dividing circuit (TMD) according to a second embodiment of thepresent invention. Since a 2-phase clock is used as the clock signal,timing division is performed using complementary signals. With thistiming dividing circuit, therefore, three intervals are required, namelya first interval over which charging is performed by one of the PMOStransistors MP1, MP2, a second interval over which charging is performedby both PMOS transistors MP1 and MP2, and a third interval over whichthe accumulated charge is discharged.

To accomplish this, as shown in FIG. 6, the output IN1.IN1 d.NAND of aNAND gate whose inputs are the signal IN1 and a signal IN1 d obtained bydelaying the signal IN1 by a delay element DL1 is connected (fed) to thegate of the PMOS transistor MP2, which forms a switch for charging thecapacitor C; and the output IN1.IN2 d.NOR (the period of the signalsIN1, IN2 is 2tCK) of a NOR gate whose inputs are the signal IN1 and asignal IN2 d obtained by delaying the signal IN2 (the time differencewhereof with respect to the signal IN1 is the period tCK) by a delayelement DL2 is connected (fed) to the gate of the PMOS transistor MP1,which forms a switch for charging the capacitor C, and to the gate of anNMOS transistor MN1, which forms a switch for discharging the capacitorC, whereby the NAND gates each generate a one-shot pulse. This producesthe first interval over which charging is performed by the one PMOStransistor MP1, the second interval over which charging is performed byboth PMOS transistors MP1 and MP2, and the third interval over which theaccumulated charge is discharged by turning on the NMOS transistor MN1.The signal waveforms associated with the circuit shown in FIG. 6 areillustrated in FIG. 7. The signal waveform A1, A2, A3 is the voltage atnode A in FIG. 6, and the output signal OUT1, OUT2, OUT3 is the outputof the buffer BUF.

Embodiment 1-3

A third embodiment will be described with reference to FIGS. 8 and 9.Averaging of timing is performed in this embodiment using a 4-phaseclock. To accomplish this, a frequency divider 603 for generating a4-phase clock is disposed in forestage of timing averaging circuits 601,602. In order to average the timings of the edges of the 4-phase clock,the timing averaging circuit is composed of timing dividing circuits TMD701-1 to 701-4 arranged in parallel in a number equal to the number ofclock phases, as shown in FIG. 9. NAND gates NAND1 to NAND4 each outputa one-shot pulse based upon every two outputs from among the outputsproduced by the timing dividing circuits TMD 701-1 to 701-4, and outputsof pairs of the NAND gates NAND1 to NAND4 are combined by NAND gatesNAND5 to NAND8, whereby a 4-phase clock of reduced jitter is produced.

Though the number of clock phases increases with this embodiment, thefact that a 4-phase clock is used makes it unnecessary to generate aone-shot pulse. This is advantageous in that it is possible to cope witha clock having a correspondingly higher frequency.

The methods of reducing jitter using a multiphase clock according to thefirst to third embodiments are applicable in a case where the number ofclock phases is two or more.

Further, since a multiphase clock is regenerated by the timing averagingcircuit, an advantage is that any number of stages can be connected inseries.

FIGS. 10 to 11 are diagrams illustrating an example of the results ofcircuit simulations illustrating the effects of jitter reductionaccording to this embodiment of the present invention. An input clock(the signal waveform at the top of FIG. 10) is composed of clock pulseshaving a period of 8 ns the timing of which is displaced once by 1 nsper four pulses. As for the eye pattern of the input pulses, the shiftof 1 ns is approximately halved by the first stage of the timingaveraging circuit and is halved again by the second stage thereof (seeFIG. 11).

Embodiment 1-4

A fourth embodiment of the present invention will now be described. FIG.12 is a diagram showing the construction of the fourth embodiment. Asshown in FIG. 12, a clock that has entered from an input buffer 1003 hasits phase delayed by a clock-cycle fixed delay circuit (a 360 degreesphase shifter) 1001 giving a delay corresponding to the clock-cycle.This delayed clock and the undelayed clock that is obtained from theinput buffer have their timings averaged by a timing averaging circuit1002, whereby jitter is reduced.

As shown in FIG. 13, the fixed delay circuit is such that the clockcycle is measured by one delay circuit 91 of a pair of delay circuitsand is reproduced by the other delay circuit 92 of the pair. At themoment the clock input to the delay circuit 91 propagates through thedelay circuit 91, which takes a length of time equal to the clock periodtCK, the next clock pulse enters, is latched by a latch circuit 93 atthis position and then is input to the delay circuit 92.

In this embodiment, the timing averaging circuit may be so adapted thatthe timings of both the rising and falling edges are averaged by thecircuitry shown in FIG. 14. The circuitry shown in FIG. 14 utilizesinternal transistors of NAND gates (MN3, MN1, MP1, etc.) and NOR gates(MN5, MP4, MP2, etc.) as parallel MOS transistors.

As shown in FIG. 14, transistors MN1, MN2 having input signals IN1, IN2connected to their gates via inverters INV1, INV2, respectively, turn onsuccessively when the high level of the input signals IN1, IN2 fall tothe low level, thereby controlling the discharging of electric charge inthe load capacitor of the commonly connected output (the gate node oftransistor MP6, which takes on the power supply potential VCC via thetransistor MP1 when the input signals IN1, IN2 are at the high level).When the gate potential of the transistor MP6 reverts to the low level,the transistor MP6 turns on and the timings of the falling edges of theoutput of inverter INV5 are averaged.

Transistors MP5, MP4 having the input signals IN1, IN2 connected totheir gates via inverters INV3, INV4, respectively, turn on successivelywhen the low level of the input signals IN1, IN2 rise to the high level,thereby charging the commonly connected output (the gate node oftransistor MN6, which takes on ground potential via the transistor MN5when the input signals IN1, IN2 are at the low level). When the gate ofthe transistor MN6 rises to the high level, the transistor MN6 turns onand the timings of the rising edges of the output of inverter INV5 areaveraged.

According to this embodiment, using the clock-cycle fixed delay circuitis convenient in that a difference in the timings of the clock pulsesinput to the timing averaging circuit can be made small, and in that thetiming averaging circuit can be realized by basic logic elements such asNAND and NOR gates. Further, using the clock-cycle fixed delay circuitis advantageous in that operating range can be widened with respect tothe clock cycle.

Further, though the amount of phase shift in the fixed delay circuit ismade 360 degrees in this embodiment, two circuits each for implementinga 180 degrees phase shift may be connected in series, as depicted inFIG. 15.

[Second Mode of the Present Invention]

A second mode for practicing the present invention will now bedescribed. FIG. 17 is a block diagram showing the construction of adelay-locked loop (DLL) according to this mode of the present invention.As shown in FIG. 17, the DLL has a timing averaging circuit 101averaging the periods between pulses of a clock signal. An externallyapplied clock used in a phase comparison and an internal clock are inputto the timing averaging circuit 101, the output of which is input to avoltage-controlled delay circuit 115.

FIG. 18 illustrates the basic operation waveforms useful in describingthe principle of operation of this mode of the present invention. In acase where a clock (2-2) containing a jitter component “a” with respectto a normal clock (2-1) enters after the DLL locks, the jitter componenthas cycles of tCK+a and tCK−a with respect to the cycle tCK of thenormal clock (2-1). The jitter component of a primary jitter-compensatedclock (2-3), which is obtained by passing the clock (2-2) with jitterthrough the timing averaging circuit 101, becomes 0.5a. Thus, the effectof this arrangement is that jitter is compensated for and reduced by acircuit for averaging the cycles of the clock.

Embodiment 2-1

A first embodiment of the second mode of the present invention will nowbe described. As shown in FIG. 17, the DLL has the timing averagingcircuit 101 averaging the cycles between pulses of the clock signal, asset forth in the description of the second mode of the presentinvention. The DLL is constituted by a charge pump 103, a loop filter104 and a voltage-controlled delay circuit 115. A clock driver 106,input buffer 107 and input buffer dummy 108 construct a feedback circuitsystem as a clock path. The outputs of the input buffer 107 and inputbuffer dummy 108 enter the timing averaging circuit 101, the output ofwhich is input to the voltage-controlled delay circuit 115. Thevoltage-controlled delay circuit 115 delivers the output of the timingaveraging circuit 101 upon variably delaying the output based upon theoutput voltage of the loop filter 104.

In this embodiment, a timing dividing circuit TMD is constructed usingparallel MOS transistors MP1, MP2, as illustrated in FIG. 5, by way ofexample. The principle of operation has already been described. Theprinciple underlying the division of timing is as shown at FIG. 4( b).That is, first the capacitor C is charged by one MOS gate for a periodtime equal to the clock period T, then the capacitor C is charged by twoMOS gates, thereby producing a time difference of T/2 in comparison witha case where the capacitor C is charged by two MOS gates from the start.

FIG. 19 illustrates an example of the construction of the timingaveraging circuit 101 used in this embodiment. As shown in FIG. 19, afour-switch group of PMOS transistors MP1, MP2, MP3, MP4 constructs aswitch for charging a load capacitance. First the PMOS transistor MP1turns on at the falling edge of the signal IN1 to perform charging atthe current i1, then the three PMOS transistors MP2, MP3, MP4 are turnedon, too, by the falling edge of the signal IN2 to charge the capacitanceby currents i1 to i4 from the four switches. Discharge takes place fromNMOS transistors MN1, MN2, MN3, MN4 when the signals IN1 and IN2 bothrise. The circuitry shown in FIG. 19 utilizes internal transistors ofNAND and NOR gates, which are basic gates.

Embodiment 2-2

A second embodiment of the second mode of the present invention will bedescribed next. FIG. 20 is a block diagram showing the construction ofthe second embodiment of the present invention. As shown in FIG. 20, thetiming dividing circuit uses a circuit arrangement identical with thatof FIG. 14 as the timing averaging circuit. Here, however, the circuitdepicted in FIG. 19 is employed as the NAND gate in the timing averagingcircuit of FIG. 14. With this circuit, the driving performance of thePMOS transistors is raised by a factor of three by the inputs IN1 andIN2. The ratio of the NMOS transistors is added on in a similar mannereven in the NOR gates. Consequently, the effects of the division oftiming also are such that the timing difference between the input of IN1and the input of IN2 is divided by four and an output having one-fourththe timing difference of IN1 relative to IN2 is obtained.

To accomplish this, the signal IN1 is coupled (fed) to the input buffer107 and the input signal IN2 is coupled to the input buffer dummy 108,whereby the jitter component of the clock input to thevoltage-controlled delay circuit 115 is reduced to one-fourth even ifthe jitter of the external clock is large.

An effect of this embodiment is that the jitter of the external clockcan be reduced to a fraction of a few'th if the ratio (the internaldividing ratio of the time difference between the two signals) taken onby the average value of the timing averaging circuit is changed.

Embodiment 2-3

A third embodiment of the second mode of the present invention will nowbe described with reference to FIG. 20. A circuit arrangement identicalwith that of the first embodiment or second embodiment of the secondmode of the present invention is used as the DLL and timing averagingcircuit. However, this embodiment has a lock sensing (detecting) circuit109 and a changeover circuit 110. According to this arrangement, whenthe DLL locks, the changeover circuit 110 changes over between the clockthat enters a phase sensing circuit 102 and the output of a timingaveraging circuit 100.

In this embodiment, the external clock no longer enters the phasesensing circuit 102 directly, as a result of which it is possible tosuppress the effects of jitter on the stable operation of the DLL.

Embodiment 2-4

A fourth embodiment of the second mode of the present invention will nowbe described. According to this embodiment, as shown in FIG. 52, thesecond mode of the present invention is applied to a PLL. Thisarrangement has the timing averaging circuit 100 for averaging thecycles between pulses of the clock signal, and the PLL is constituted bythe phase sensing circuit 102, charge pump 103, loop filter 104 andvoltage-controlled oscillator (VCO) 105. The clock driver 106, inputbuffer 107 and input buffer dummy 108 construct a feedback circuit as aclock path. The outputs of the input buffer 107 and input buffer dummy108 are input to the timing averaging circuit 100 and the output of thetiming averaging circuit 100 enters the phase sensing circuit 102. Inthis embodiment, the circuit arrangement shown in FIG. 19, for example,is used as the timing averaging circuit 100. That is, this embodimenthas a lock sensing circuit 109 and a changeover circuit 110. When thePLL locks, the changeover circuit 110 changes over between the clockthat enters the phase sensing circuit 102 and the output of a timingaveraging circuit 100. In this embodiment, the external clock no longerenters the phase sensing circuit 102 directly, as a result of which itis possible to suppress the effects of jitter on the stable operation ofthe PLL.

[Third Mode of the Present Invention]

FIG. 30 shows the construction of a synchronous delay circuit accordingto this mode for practicing the present invention, and FIG. 31 is atiming chart useful in describing the principle of operation of a timingaveraging circuit. This mode of the present invention has a timingaveraging circuit 1001 (the construction of which is as shown in FIG. 3)which averages the cycles between pulses of the clock signal. Anexternal clock input that enters from an input buffer 903 and a signalobtained by passing an internal clock signal 907, which is output by aclock driver 904, through an input buffer dummy 905A having a delay timecharacteristic equivalent to that of the input buffer 903 are input tothe timing averaging circuit 1001. The output of the timing averagingcircuit 1001 is input to a delay circuit 901 via a dummy delay circuit905. At the moment the signal input to the delay circuit chain (orarray) 901 advances by an amount equivalent to the clock cycle, thesignal is transferred through the delay circuit chain 902 by the nextclock pulse and exits from the delay circuit chain 902 upon traveling adistance equivalent to the distance traversed in the delay circuit chain901. It should be noted that the dummy circuits of the input buffer areserially connected in the dummy delay circuit.

The principle of operation of this mode will be described with referenceto the timing chart of FIG. 31. In a case where a clock 2-2 containing ajitter component “a” with respect to a normal clock 2-1 enters after thesynchronous delay circuit locks, the jitter component has cycles oftCK+a and tCK−a with respect to the cycle tCK of the normal clock. Thejitter component of a primary jitter-compensated clock 2-3, which isobtained by passing the clock 2-2 with jitter through the timingaveraging circuit, becomes 0.5a. Thus, the effect of this arrangement isthat the jitter is compensated for and reduced by a circuit thataverages the cycles of the clock.

Embodiment 3-1

FIG. 32 is a diagram showing the construction of a first embodiment ofthe third mode of the present invention. This embodiment supplies theaveraging circuit of the present invention to a basic part as shown inFIG. 24. In this embodiment, use is made of two of the timing averagingcircuits 1001 (these timing averaging circuits each comprise the timingdividing circuit of FIG. 3) to which two signals are input and whichoutput signals having a time difference obtained by internally dividingthe time difference between the two input signals. Both timing averagingcircuits receive outputs of the input buffer 903 and input buffer dummy905A as inputs. The first timing averaging circuit 1 applies its outputto the delay circuit chain (or array) 901, and the output of the secondtiming averaging circuit 2 is used as a signal (a signal for decidingdelay length) for controlling transfer of a signal from the delaycircuit chain 901 to the delay circuit chain 902. The timing averagingcircuits 1001 are basically identical in construction and are capable ofbeing set separately to averaging ratios (internal-division ratios) ofthe two input signals. The averaging or internal-division ratio of thetwo input signals can be set by deciding the ratio of the sizes (e.g.,transistor widths) of the transistors that receive the two input signalsin the timing averaging circuits.

For more detail of the basic part shown in FIG. 24, refer to thedisclosure of the copending U.S. patent application Ser. No. 09/087,864filed on Jun. 1, 1998. Generally, the same applies to the basic partsshown in FIGS. 23–29, too.

By separately setting the averaging ratios or internal-division ratiosof the two timing averaging circuits, it is possible to adjust thephases of the outputs and suppress the occurrence of a phase error owingto the provision of two timing averaging circuits.

Embodiment 3-2

The construction of a second embodiment of the third mode of the presentinvention is illustrated in FIGS. 33 and 34. This embodiment applies thetiming averaging circuit of the present invention to a basic partillustrated in FIG. 25. Two of the timing averaging circuits 1001 areused in this embodiment as well. In the arrangement illustrated in FIG.34, the clock from the second timing averaging circuit 2 isfrequency-divided by a divide-by-two frequency divider circuit 24, thefrequency-divided clock signal and its complementary signal are suppliedas signals that decide the delay lengths of the delay circuit chains901, 902, and the outputs of the delay circuit chains 901, 902 arecombined and extracted by a combiner 20A. The resulting signal issupplied from the clock driver 904 as an internal clock. In thisembodiment, continuous operation is implemented by using the prior-artcircuit of FIG. 25 in alternating fashion.

Embodiment 3-3

FIG. 35 illustrates the construction of a third embodiment of the thirdmode of the present invention. This embodiment applies the timingaveraging circuit of the present invention to the basic part illustratedin FIG. 26. Two of the timing averaging circuits 1001 are used in thisembodiment as well. The clock from the second timing averaging circuit 2is frequency divided by the divide-by-two frequency divider circuit 24,the frequency-divided clock signal and its complementary signal aresupplied as signals that decide the delay lengths of the delay circuitchains 901, 902, and the outputs of the delay circuit chains 901, 902are combined and extracted by the combiner 20A. The resulting signal issupplied from the clock driver 904 as the internal clock.

Embodiment 3-4

FIG. 36 illustrates the construction of a fourth embodiment of the thirdmode of the present invention. This embodiment applies the timingaveraging circuit of the present invention to the basic part illustratedin FIG. 27. Two of the timing averaging circuits 1001 are used in thisembodiment as well. The output of the first timing averaging circuitenters the delay circuit chain 901 from the dummy delay circuit 905, andthe output of the second timing averaging circuit is used as a signal (asignal for deciding delay length) for controlling a latch that controlstransfer of a signal from the delay circuit chain 901 to the delaycircuit chain 902.

[Fourth Mode of the Present Invention]

A fourth mode of the present invention will now be described. FIG. 37 isa diagram showing the construction of a fourth mode for practicing thepresent invention. As shown in FIG. 37, the synchronous delay circuit ofthe present invention has a pair of delay circuit chains 11, 12 and amonitor signal generating circuit 19 for sensing the amount of delay inthe clock path. (The details of a synchronous delay circuit using amonitor signal generating circuit are described in the specification ofU.S. patent application Ser. No. 09/087,864 claiming the priority ofJapanese Patent Application No. 9-157974 filed by the present inventor.Now the entire disclosure thereof is incorporated herein.)

In terms of the operation of this circuit, a delay quantity[tCK−(td1+td2)] obtained by subtracting the amount of delay of the clockpath from an amount of delay equivalent to one cycle using one of thedelay circuit chains and the delay-quantity sensing circuit is measuredas the length of the delay circuit traversed by a signal, and use ismade of the other delay circuit chain and the original clock path togenerate a clock signal having a phase equal to that of the externalclock. Furthermore, the output of the timing averaging circuit 1001,which is for averaging the cycles between pulses of the clock signal, isinput to the delay circuit chain and the monitor signal generatingcircuit 19.

The operation through which a clock having a phase identical with thatof the external clock is generated will be described with reference tothe circuit arrangement shown at (a) in FIG. 37 and the timing chargeshown at (b) of FIG. 37.

This arrangement has a synchronous delay circuit constituted by a delaycircuit chain 911 for measuring a fixed time, and a delay circuit chain912 for reproducing the measured delay time, as well as an input buffer13 and a clock driver 14. The period of the clock is measured by passinga signal through the delay circuit chain 911 for measurement. However,in an interval that causes clock skew, such an interval over which theclock pulse is traveling through the clock driver 14 and an intervalover which the clock pulse passes through an input buffer dummy 15, nosignal travels through the delay circuit chain 911, thereby haltingmeasurement of the clock period tCK.

Accordingly, the interval over which the signal advances through thedelay circuit chain 911 becomes exactly tCK−(td1+td2), which is obtainedby subtracting the delay time td2 of the input buffer and the delay timetd1 of the clock driver from the clock period tCK, and the delay timereproduced by the delay circuit chain 912 also becomes tCK−(td1+td2). Asa result, exactly one clock is required for a clock pulse to passthrough the input buffer 13, delay circuit chain 912 and clock driver14, and skew with respect to the external clock is essentiallyeliminated.

The monitor signal generating circuit 19, which generates a signal formonitoring the period of time over which a pulse advances through theclock driver 14 and input buffer dummy 15, is constituted by anRS-(reset-set) flip-flop, etc. Accordingly, in a case where a length oftime which is the sum of the delay quantities of the clock driver 14 andinput buffer dummy 15 is greater than the clock period tCK, the passageof the signal through the delay circuit 911 for measuring the delay ofthe clock driver and input buffer can be suspended for a time equal tothe excess. In other words, clock skew can be eliminated also in a casewhere the sum of the delays of the clock driver 14 and input bufferdummy 15 is greater than the clock period tCK.

A monitor signal D which halts the advance of a clock signal as shown inthe timing chart of FIG. 37( b) monitors the interval over which a clockpulse passes through the clock driver. The timing at which a clockadvances through the clock driver at synchronization, therefore, ishastened by tCK−(td1+td2).

Accordingly, at the changeover from the state before synchronization tothe state after synchronization, the monitor signal D is issued twice inone cycle (i.e., as shown by hatched pulse and a subsequent pulse in thetiming chart). However, this is avoided by doing the following after thefirst output of the internal clock or after the output of the firstmonitor signal D:

(1) halting the monitor signal D indicated by the hatching in the timingchart of FIG. 37( b), or

(2) halting the monitor signal D indicated by the hatching in the timingchart of FIG. 37( b) through a method such as changing over the clockpath to stop the advance of a pulse to the clock driver.

In this embodiment, clock skew can be eliminated at a timing one cycleearlier than in the first embodiment because merely a clock signalemerges for which only once skew has not been removed.

The timing averaging circuit 1001, which has a construction similar tothat of each of the foregoing embodiments, averages the timings of theoutput of the external clock and internal clock from the input buffer 13and input buffer dummy 14 and applies its output signal to the delaycircuit chain. Further, whether the output of the timing averagingcircuit 1001 is supplied to the delay circuit chain 911 is determined bya changeover unit 10B, and averaging of timing is performed from themoment the internal clock is output via the delay circuit chain.

Embodiment 4-1

FIG. 38 illustrates the construction of a first embodiment of the fourthmode of the present invention. As shown in FIG. 38, this embodimentapplies the averaging circuit of the present invention and thearrangement of the fourth embodiment, which comprises the art forstopping advance of the clock through the synchronous delay circuit bythe monitor signal generating circuit, to the basic part shown in FIG.24. According to this embodiment, two of the timing averaging circuitsare used, both the first and second timing averaging circuits 1 and 2receive the output of the input buffer 13 and the output of the inputbuffer dummy 15 as inputs, the output of one of the timing averagingcircuits enters the delay circuit chain 11 and the output of the delaycircuit is used as a signal for transferring a signal from the delaycircuit chain 11 to the delay circuit chain 12. The two timing averagingcircuits are basically identical in construction and are capable ofbeing set separately to averaging ratios (internal-division ratios) ofthe two input signals. The averaging ratio or internal-division ratio ofthe two input signals can be set by deciding the ratio of the sizes(e.g., transistor widths) of the transistors that receive the two inputsignals in the timing averaging circuits.

By separately setting the averaging ratios or internal-division ratiosof the two timing averaging circuits, it is possible to suppress theoccurrence of a phase error owing to the provision of two timingaveraging circuits.

In this embodiment also the arrangement is such that whether a timingaveraging circuit is used or not is decided by a changeover unit (notshown). Timing averaging is carried out from the moment the internalclock is output via the delay circuit.

Embodiment 4-2

FIG. 39 illustrates the construction of a second embodiment of thefourth mode of the present invention. This embodiment applies thearrangement of the fourth mode to the basic part illustrated in FIG. 25.Two timing averaging circuits are used in this embodiment as well. Inthe arrangement shown in FIG. 39, continuous operation is implemented byfrequency dividing the clock and using the basic part circuits of FIG.25 in two sets in alternating fashion.

The outputs of the first and second timing averaging circuits 1 and 2enter the divider circuit 24, which internally selects or logicallycombines these signals, and the appropriate timing can be selected fromthe outputs of the two timing averaging circuits at the rising edge andfalling edge of the frequency-divided signal. This selection of timingdepends upon the particular case, namely whether reduction of externaljitter or reduction of phase error takes precedence. A description isnot given here. The selection circuit per se and the frequency dividercircuit per se can be implemented by the usual circuit designs. Theoutput of the delay circuits 12 of the two delay circuit chains sets isselected by changeover unit 10B and input to a changeover unit 10A, whenthe signal is supplied to the clock driver 14.

Embodiment 4-3

FIG. 40 illustrates the construction of a third embodiment of the fourthmode of the present invention. This embodiment applies the fourth modeto the basic part shown in FIG. 26. Two timing averaging circuits areused in this embodiment as well.

Embodiment 4-4

FIG. 41 illustrates the construction of a fourth embodiment of thefourth mode of the present invention. This embodiment applies the fourthmode to the basic part shown in FIG. 27. Two timing averaging circuitsare used in this embodiment as well. The outputs of the first and secondtiming averaging circuits 1, 2 are frequency divided by thedivide-by-two frequency divider circuit 24 and are used as signals thatdecide the input and delay length of the delay circuit 11 of the twodelay circuits. The output of the delay circuit 12 is selected by thechangeover unit 10B and input to the changeover unit 10A, when thesignal is applied to the clock driver 14.

[Fifth Mode of the Present Invention]

A fifth mode for practicing the present invention will now be described.As shown in FIG. 43, the synchronous delay circuit according to thefifth mode of the present invention has a plurality of the timingaveraging circuits for averaging the cycles between pulses of the clocksignal. The external clock signal from the input buffer 13 and theinternal clock signal from the input buffer dummy 15 enter each of thetiming averaging circuits 1, 2, and the outputs of the timing averagingcircuits 1, 2 are connected to the delay circuit chains 11, 12 and tothe monitor signal generating circuit 19. The timing averaging circuits1, 2 are basically identical in construction and are capable of beingset separately to averaging ratios (internal-division ratios) of the twoinput signals. The averaging ratio or internal-division ratio of the twoinput signals can be set by deciding the ratio of the sizes (e.g.,transistor widths) of the transistors that receive the two input signalsin the timing averaging circuits.

By separately setting the averaging ratios or internal-division ratiosof the two timing averaging circuits 1, 2, it is possible to suppressthe occurrence of a phase error owing to the provision of two timingaveraging circuits.

Embodiment 5-1

FIG. 43 illustrates the construction of a first embodiment of a fifthmode for practicing the present invention. This embodiment applies thefifth mode of the present invention to the basic part illustrated inFIG. 24. The embodiment has three timing averaging circuits 1, 2, 3 eachof which receives the output of the delay circuit chain 12 and output ofthe input buffer dummy 15 as inputs. The output of the timing averagingcircuit 1 is input to the delay circuit chain 11, the output of timingaveraging circuit 2 is used as a signal for transferring a signal fromthe delay circuit chain 11 to the delay circuit chain 12, and the outputof the timing averaging circuit 3 is input to the monitor signalgenerating circuit 19.

Whether a timing averaging circuit is used or not is decided by achangeover unit (not shown). Timing averaging is carried out from themoment the internal clock signal is output from the clock driver 14 viathe delay circuit chains 11, 12.

Embodiment 5-2

FIG. 44 illustrates the construction of a second embodiment of a fifthmode for practicing the present invention. This embodiment applies thefifth mode of the present invention to the basic part illustrated inFIG. 25. The embodiment also uses three timing averaging circuits 1, 2,3. In the arrangement depicted in FIG. 44, continuous operation isimplemented by frequency dividing the clock by the frequency divider 24and using the two delay circuit chains, which are circuits exemplifyingthe basic part circuits of FIG. 25, in two sets in alternating fashion.

The outputs of the two timing averaging circuits 1 and 2 enter thedivider circuit 24, which internally selects or combines these signals,and the appropriate timing can be selected from the outputs of the twotiming averaging circuits at the rising edge and falling edge of thefrequency-divided signal. This selection of timing depends upon theparticular case, namely whether reduction of external jitter orreduction of phase error takes precedence. A description is not givenhere. The changeover circuit per se and the frequency divider circuitper se can be implemented by the usual circuit designs and are notdescribed here.

Embodiment 5-3

FIG. 45 illustrates the construction of a third embodiment of the fifthmode of the present invention. This embodiment applies the fifth mode tothe basic part shown in FIG. 26. Three timing averaging circuits areused in this embodiment, too. In this embodiment also the output of thefrequency divider 24, which frequency divides the outputs of the timingaveraging circuits 1, 2, is applied to each of the delay circuit chains,and continuous operation is implemented by using the delay circuitchains in two sets in alternating fashion via the changeover unit 10B.

Embodiment 5-4

FIG. 46 illustrates the construction of a fourth embodiment of the fifthmode of the present invention. This embodiment applies the fifth mode tothe basic part shown in FIG. 27. Three timing averaging circuits areused in this embodiment, too. The output of the input buffer 13 and asignal obtained by passing the internal clock 17 through the inputbuffer dummy 15 enter each of the timing averaging circuits 1, 2, 3. Theoutput of the timing averaging circuit 1 is applied to the delay circuitchain 11, the output of the timing averaging circuit 2 is supplied as asignal for controlling transfer from the delay circuit chain 11 to thedelay circuit chain 12, and the output of the timing averaging circuit 3is applied to the monitor signal generating circuit 19.

[Sixth Mode of the Present Invention]

A sixth mode for practicing the present invention will now be described.As shown in FIG. 47, a timing averaging circuit for averaging the cyclesbetween pulses of the clock signal is not expressly shown in the form ofa circuit block. The timing averaging circuits are incorporated withinthe delay circuit chains. As shown in FIG. 50, the delay circuit chainsthemselves are constructed by arraying gates in parallel and function astiming averaging circuits.

More specifically, as shown in FIG. 50, the first and second delaycircuit chains 11, 12 have, as a delay circuit of one stage, first andsecond clocked inverter circuits (MN11, MP11, MN11C, MP11C and MN12,MP12, MN12C, MP12C) arranged in parallel with respect to the inputsignals, output nodes (FIn+1b, FIn+1a) to which first and second clockdriver circuits are commonly connected are charged and discharged, andeach node (FIn+1b, FIn+1a) of the first delay circuit chain 11 isconnected to the corresponding node (BIn+1b, BIn+1a) of the second delaycircuit chain 12. In the arrangement shown in FIG. 50, a PMOS transistorMP11 and an NMOS transistor MN11 of the clocked inverter are connectedto the power supply and ground, respectively, at locations closer to thepower supply and ground than transistors MP11C, MN11C, which are forcontrolling activation and deactivation. That is, the transistors MP11C,MN11C for controlling activation and deactivation are disposed betweenthe power supply and ground at positions further inward than the CMOStransistors for signal drive.

As shown in FIG. 47, the external clock from the input buffer 13 and asignal obtained by passing the internal clock through the input bufferdummy 15 are frequency divided by the divide-by-two frequency dividercircuits 24 and then applied to the delay circuit chain 11 as its twoinputs. Transistors MP11C, MN11C for switch control are turned on andoff by the frequency-divided signals from the frequency dividers 24 ofFIG. 47.

The parallel delay circuit chains are basically identical inconstruction and the averaging ratios (internal-division ratios) arecapable of being set independently depending upon the ratio of the sizesof the transistors (e.g., ratio of gate widths of the transistors).

In this embodiment, the clock path is not occupied solely for the sakeof timing averaging. As a result, advantages are that there is nolimitation imposed upon high-speed operation by a timing averagingcircuit and that a timing difference between the external clock andinternal clock is not limited to the delay time difference of the timingaveraging circuit.

Embodiment 6-1

FIG. 48 illustrates the construction of a first embodiment of the sixthmode for practicing the present invention. According to this embodiment,the timing averaging circuits are incorporated within the delaycircuits, as shown in FIG. 50, and the delay circuits themselves areconstructed by arraying gates in parallel and function as timingaveraging circuits. As shown in FIG. 48, the external clock from theinput buffer 13 and a signal obtained by passing the internal clocksignal 17 through the input buffer dummy 15 are frequency divided by thedivide-by-two frequency divider circuit 24 and then applied as two inputsignals to each delay circuit chain 11, 11′ in each of two sets of thedelay circuit chains and also input as a signal for deciding the delaylength. The appropriate timing can be selected by the changeover unit10B from the outputs (2 sets) of the two timing averaging circuits atthe rising edge and/or falling edge of the signal frequency divided bythe frequency divider 24. This selection of timing depends upon theparticular case, namely whether reduction of external jitter orreduction of phase error takes precedence. A description is not givenhere. The changeover circuit per se and the frequency divider circuitper se can be implemented by the usual circuit designs and are notdescribed here.

FIG. 49 illustrates the construction of a second embodiment of the fifthmode of the present invention. As in the arrangement shown in FIG. 48,in this embodiment the timing averaging circuits are incorporated in thedelay circuit chains 11, 12. This arrangement differs from that shown inFIG. 48 in that it is equipped with two of the monitor signal generatingcircuits 19 for respective ones in each set of the delay circuit chains.The monitor signal generating circuit 19 generates the monitor signalfor halting the advance of the clock through the delay circuit chain 11for a period of time equivalent to the delay times of the clock driverand input buffer dummy.

Though this mode and its embodiments use the basic part of FIG. 25 as abase, similar effects are obtained if the basic parts shown in FIGS. 24,27 and 28 are adopted as the base.

Further, the delay circuit chains 11, 12 are constructed in units ofdelay circuits composed of clocked inverters. Among the seriallyconnected transistors of the delay circuits, the transistors of theclocking gates are disposed on the output side, and timing averaging andcontrol of clock signal advance are given a higher priority than passagethrough the delay circuits.

Part of one of the delay circuit chains shown in FIG. 50 can beextracted and utilized as a timing averaging circuit of a synchronousdelay circuit.

In addition, in order to reduce the number of elements in the delaycircuit chains, the delay circuit chains shown in FIG. 50 may beconstructed by adopting an arrangement in which the PMOS transistors andNMOS transistors are repeated in one chain in alternating fashion. Suchan arrangement is shown in FIG. 51 and reduces the number of elements byapproximately half.

In FIG. 51, the delay circuit chain 11 has first and second PMOSswitches MP11, MP12 in one stage for being turned on by the fallingedges of the first and second input signals. These first and second PMOSswitches MP11, MP12 are turned on in response to the falling edges ofthe first and second input signals, whereby the common output nodes ofthese first and second PMOS switches are charged. The commonly connectednodes are input to the first and second NMOS switches MN21, MN22 of thenext stage. These first and second NMOS switches MN21, MN22, of the nextstage are turned on in response to the rising edge of the commonlyconnected nodes, thereby discharging the output nodes to ground. Thedelay circuit chain 12, through which signals propagate in a directionopposite that of the delay circuit chain 11, has PMOS switches MP13,MP14 and NMOS switches MN23, MN24 in the stages that correspond to thePMOS switches MP12, MP13 and NMOS switches MN21, MN22 of each stage ofthe delay circuit chain 11. The output node of the PMOS switch stage ofdelay circuit chain 11 is connected to the output node of the NMOSswitch in delay circuit chain 12 of a stage thereof preceding the PMOSswitch which corresponds to the above-mentioned stage. A switch isinserted between each PMOS switch and the power supply VCC and betweenthe NMOS switch and the output in order to control the ON/OFF (closingand opening) of these paths.

In order to deal with long cycles, the delay circuits may be makering-shaped and the number of times signals circulate through the ringsmay be controlled by a counter (see the specification of Japanese PatentKokai Publication No. 8-137091).

Effects of the Invention will be briefly mentioned as follows.

In accordance with the present invention, as described above, anaveraging circuit is provided for generating a signal having a timecomponent obtained by averaging an input time difference between twosignals input with certain constant time difference between them, andpulses of a different clock signal are supplied to this circuit, therebygenerating a clock signal in which the time difference between thedifferent pulses is internally divided. As a result, clock signal jittercan be reduced without using a feedback circuit such as a PLL.

Further, in accordance with the present invention, the effect ofreducing jitter can be doubled or enhanced by connected timing averagingcircuits in series.

In accordance with the present invention, use is made of a multi phaseclock obtained by frequency dividing a clock signal. This makes itpossible to use a timing averaging circuit in which MOS transistors aresimply arranged in parallel.

By increasing the number of phases of the multi phase clock, it ispossible to cope with and support a high-frequency clock signal.

Furthermore, in accordance with the present invention, a timingaveraging circuit can be realized through a simple arrangement, whichemploys NAND and NOR gates, by using fixed-delay circuit chain.

In accordance with the present invention, there is provided a systemhaving a circuit for generating a signal having a time componentobtained by averaging an input time difference between two signals inputwith certain constant time difference between them, and a circuit forapplying pulses of a different clock signal to this circuit, wherein aDLL circuit is equipped with a circuit for internally dividing the timedifference between these different pulses, an external clock input andan internal clock input used in a phase comparison are applied to thecircuit which internally divides the time difference between thedifferent pulses, and the output of this circuit is input to avoltage-controlled delay circuit. As a result, jitter of the clocksignal is averaged by internal division with the internal clock, wherebyjitter is reduced.

Further, in accordance with the present invention, when clock signaljitter is averaged by internal division with the internal clock, theinternal-division ratio is decided so as to enlarge the timing componentof the internal clock, thereby doubling/enhancing the jitter reducingeffect.

In accordance with the present invention, a signal input to a phasecomparator is changed over from the external clock to a signal obtainedby internally dividing the external clock and the internal clock, withthe changeover being made after locking is achieved. This reduces theinfluence of the external clock jitter on the stable operation of a DLLor PLL.

Furthermore, in accordance with the present invention, a synchronousdelay circuit is equipped with an averaging circuit, thereby reducingthe jitter component of the internal clock.

As many apparently widely different embodiments of the present inventioncan be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificembodiments thereof except as defined in the appended claims.

It should be noted that any further objects and aspects of the presentinvention will become apparent from the entire disclosure, other thanthose explicitly recited herein.

Also it should be noted any combination of anyelements/components/units/modules disclosed in the embodiments oromission of any part therefrom may be done within the generic concept ofthe present invention.

1. A timing circuit comprising: frequency dividing means which divides afrequency of a timing signal and outputs a first signal having a firstphase and a second signal having a second phase; parallel circuits whicheach receive the first and second signal and generate an output signalat a time referenced from a transition of the earlier occurring signalof the first and second signals, and wherein the output signalgeneration time is determined by a time difference between a low-to-hightransition of the first signal and a low-to-high transition of thesecond signal or between a high-to-low transition of the first signaland a high-to-low transition of the second signal; and means forcombining the output signals from the parallel circuits to recover thefrequency of the timing signal.
 2. The timing circuit of claim 1 whereinthe parallel circuits comprise a stage and a plurality of stages areserially connected.
 3. A timing circuit comprising: a frequency dividingcircuit which divides a frequency of a timing signal and outputs a firstsignal having a first phase and a second signal having a second phase;parallel circuits which each receive the first and second signal andgenerate an output signal at a time referenced from a transition of theearlier occurring signal of the first and second signals, and whereinthe output signal generation time is determined by a time differencebetween a low-to-high transition of the first signal and a low-to-hightransition of the second signal or between a high-to-low transition ofthe first signal and a high-to-low transition of the second signal; anda multiplexing circuit which combines the output signals from theparallel circuits to recover the frequency of the timing signal.
 4. Thetiming circuit of claim 3 wherein the parallel circuits comprise a stageand a plurality of stages are connected in series.